The present invention relates to triphenylamine dimer derivatives useful for materials for electrophotographic photoreceptors, organic electroluminescent (EL) materials and the like. Particularly, the present invention relates to a mixed substance of triphenylamine dimers capable of controlling its crystallization during film formation when it is used for a thin film of a charge-transporting layer of a layered type electrophotographic photoreceptor.
Triphenylamine dimer (TPD) derivatives have been used as materials for electrophotographic photoreceptors, organic electroluminescent (EL) materials and the like. Particularly, they are widely employed as charge-transporting materials (CTM) of organic photoreceptors for electrophotography such as copy machines, printers and the like or hole transporting materials (HTM) in EL devices.
In the early stage of research, 4,4-TPD obtained by using 4-methyldiphenylamine as a raw material was used as such a TPD derivative. However, there is a problem that when this is contained practically in a thin film of a charge-transporting layer, crystallization is liable to occur due to symmetrical structure of 4,4-TPD.
When a TPD derivative is used as a component of a layered type photoreceptor, both the electric property of the TPD itself and the film-forming property thereof are both important properties for getting the best electric property. If crystals of TPD are deposited in a film during a drying step and uniformity of the film is lost, the electric property as a photoreceptor is deteriorated obviously.
To solve the problem of poor film forming property of 4,4-TPD, 3,3-TPD obtained by using 3-methyldiphenylamine as a raw material has come to be used widely. That is, by introducing the methyl group into TPD at the m-position, symmetry of 3,3-TPD (compound) is reduced to prevent crystallization when a solvent dries. Further, addition of a small amount of 4,4-TPD to 3,3-TPD is also made for that purpose.
On the other hand, for manufacturing TPD simply in high yield, one of the present inventors has developed a novel method for manufacturing 3,3-TPD (Japanese Patent Laid-Open Publication No. 2000-256276 and U.S. Pat. No. 6,242,648). The 3,3-TPD obtained in this method has high purity and is excellent in electric property as a compound.
However, there has become apparent a new problem in that as the purity of 3,3-TPD becomes higher, crystallinity of the compound also becomes higher unfortunately and if this compound is practically contained in a thin film of a charge-transporting layer, film-forming property is deteriorated.
In other words, the reason why crystallization during film formation has conventionally been controlled by using 3,3-TPD probably is not the substitution with methyl at the 3-position, but probably is that the amine employed as a raw material for condensation, is a mixture of 3-methyldiphenylamine, 3,3xe2x80x2-dimethyldiphenylamine, diphenylamine and the like, and the conventional 3,3-TPD contains a certain amount of TPD analogues by-produced from them. On the other hand, these TPD analogues deteriorate electric properties of an electrophotographic photoreceptor as impurities.
The objective of the present invention is to provide a TPD derivative being excellent in electric properties as a compound due to its extremely less content of impurities (TPD analogues) as well as being excellent in film-forming property due to its low crystallinity.
The present invention provides a mixed substance of TPDs which comprises at least the compound represented by formula B. 
The term xe2x80x9cmixed substance of TPDsxe2x80x9d used in this specification refers to a mixture substantially composed of positional isomers of TPD represented by the chemical formula. Thus, for example the above-mentioned TPD analogs are impurities of the mixed substance of TPDs of the present invention, and it is desirable that the content thereof is as small as possible.
A mixed substance of TPDs of the present invention preferably contains the compound represented by formula A, 
the compound represented by formula B, 
and the compound represented by formula C. 
The mixed substance of TPDs of the present invention is excellent in electric properties as a charge-transporting material and has low crystallinity. Thus, when a charge-transporting layer of a layered type electrophotographic photoreceptor is formed by using the mixed substance, a uniform, non-crystalline thin film is provided (see FIG. 2) and a photoreceptor of high sensitivity is provided.